What is the Hp/A section factor?

Hp/A is the ratio of the heated perimeter (Hp) to the cross-sectional area (A) of a steel section, measured in m⁻¹. It indicates how quickly a section heats up in a fire — higher Hp/A means faster heating and more protection needed. Light sections (e.g., 203x102x23 UB, Hp/A = 230) need more protection than heavy sections (e.g., 305x305x97 UC, Hp/A = 95).

What fire rating does my building need?

Fire ratings are specified in Building Regulations Approved Document B. Typical requirements: 30 minutes for small offices (single storey), 60 minutes for most multi-storey commercial buildings, 90 minutes for buildings over 30m in height, 120 minutes for basements and special risk areas. Check with Building Control for your specific building.

What is intumescent paint and when should I use it?

Intumescent paint is a thin coating (0.5-5mm DFT) that swells to form an insulating char layer when exposed to fire. It is used where the steel must remain visible (architectural steelwork). Thin film intumescent provides up to 60 minutes protection; thick film up to 120 minutes. It costs more per m² than board or spray but looks better.

What is the difference between board and spray fire protection?

Board protection (e.g., Promat, Supalux) uses rigid boards mechanically fixed around the steel. It gives a neat boxed appearance and can be plastered/painted. Spray protection (e.g., vermiculite/cement mix) is sprayed directly onto the steel surface. Spray is cheaper per m² and faster to apply but gives a rough finish. Board is used in visible areas; spray in hidden voids.

Can I reduce fire protection by using thicker steel sections?

Yes, heavier steel sections have lower Hp/A values, meaning they heat up more slowly and need less protection (or may achieve some fire resistance unprotected). In some cases, specifying a heavier beam can eliminate the need for applied fire protection at 30 minutes. This is called 'fire engineering' and should be assessed by a fire engineer.

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Fire Protection Steel Calculator — Hp/A Factor & Protection Thickness

Calculate fire protection requirements for structural steel: Hp/A section factor, protection thickness by fire rating, and coverage area for board, spray, or intumescent systems.

Steel Section

Section determines the Hp/A factor

Fire Rating Required

From Building Regulations Approved Document B

Protection Type

Select based on finish requirements and budget

Member Length

Length of each steel member

How We Calculate This

This calculator determines fire protection requirements for structural steel members based on the Hp/A section factor, required fire rating, and protection system type.

Key concepts

Hp/A factor: Heated perimeter / cross-sectional area (m⁻¹)
Protection thickness: Increases with Hp/A and fire rating
Intumescent: 0.5-5mm DFT, swells in fire, aesthetic finish
Board: 15-50mm rigid boards, neat boxed appearance
Spray: 10-45mm sprayed coating, cost-effective for hidden steel

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.